The adhesive interactions of cells with other cells and between cells and the extracellular matrix are believed to play critical roles in a wide variety of processes including, for example, modulation of the immune system, regulation of developmental processes and tumor progression and metastasis. These interactions are mediated by adhesion molecules which transduce information from the extracellular to the intracellular matrix.
Three families of adhesion molecules which mediate these interactions have been identified: the integrins, the cadherins and the selections. In general, adhesion molecules are transmembrane proteins which contain an extracellular domain for interacting with an extracellular matrix or cellular component, a transmembrane domain spanning the cell membrane and a cytoplasmic domain for interacting with one or more cytoskeletal components.
The integrins represent one of the best characterized families of adhesion receptors. Integrins are glycoprotein heterodimers which contain a noncovalently-associated α and β subunit. There are fourteen known α subunits and eight known β subunits which can pair to form at least twenty different integrin molecules. Several distinct integrin α chains are capable of pairing with one type of β chain to form a β chain subfamily.
Recently, Parker et al. described a novel integrin heterodimer that is expressed on intra-epithelial T lymphocytes (iIEL), i.e., the population of T lymphocytes located along the basolateral surfaces of the epithelial cells which line the mucosa, adjacent to the epithelial cell basement membrane. (Parker, C. et al. (1992) Proc. Natl. Acad. Sci. USA 89:1924). Originally defined by an antibody which recognizes the human mucosal lymphocyte 1 antigen (HML-1), the novel integrin is present on >90% of intestinal IEL (iIEL) and on approximately 40% of lamina propria T lymphocytes (which lie between the epithelial basement membrane and the muscularis mucosae) (Cerf-Bensussan, N. et al., (1987) Eur. J. Immunol. 17, 1279-1285). The HML-1 antigen contains a novel α chain (designated αE, for “epithelial associated”) associated with a β7 chain (Parker, C. et al. (1992) Proc. Natl. Acad. Sci. USA 89:1924). Although the αE and β7 subunits have been cloned, a receptor for the αEβ7 integrin has not been identified. (See U.S. patent application Ser. No. 08/199,776, the contents of which are incorporated herein by reference, which discloses the primary structure of the αE chain and Yuan Q. et al. (1990) Int. Immunol. 2:1097; Erle, D. J. et al., (1991) J. Biol. Chem. 266, 11009-11016, which disclose a HML-1 β7 chain clone).
The cadherins play an important role in the establishment and maintenance of intercellular connections between cells of the same type (reviewed in Geiger B. et al. (1992) Annual Review of Cell Biology 8:307; Kemler R. (1993) Trends in Gastroenterology 9:317; Takeichi M. (1990) Annual Review of Biochem. 59:237; Takeichi M. (1991) Science 251:1451). The cadherins are synthesized as precursors that are cleaved during post-translational processing. The mature cadherins are single chain molecules which include a relatively large extracellular domain (typically divided into five sections or “ectodomains”), a single transmembrane region and a cytoplasmic tail. Sequence analysis of cadherin cDNA clones reveals that the extracellular cadherin amino acid sequence can be divided into three inter-homologous ectodomains (“EC 1-3”), each of which is 110 amino acids in length (Takeichi M. (1990) Annual Review of Biochem. 59:237). Within the ectodomains, characteristic sequences of four to five amino acids (LDRE and DXNDN) are well conserved among all cadherins. In particular, the sequence DXNDNXP, present in EC 1-3, is thought to bind divalent calcium and is generally believed to be essential for cadherin function. Two additional, less well conserved domains (“EC 4-5”) are located proximal to the membrane. Among the classical cadherins (i.e., P—(placenta), E—(epithelial), and N—(neural) cadherin), the cytoplasmic domain contains the highest degree of homology, followed by the EC-1 domain (Takeichi M. (1990) Annual Review of Biochem. 59:237). The high degree of homology observed for the cytoplasmic domain reportedly is a reflection of the association of cadherins with a group of intracellular proteins, “the catenins”, that stabilize cadherin active conformation (Kemler R. (1993) Trends in Gastroenterology 9:317).
It is generally believed that sequences in the EC-1 extracellular domain are necessary to mediate homotypic (i.e., cadherin-to-cadherin) binding. Swapping experiments in which part of the E-cadherin molecule is replaced with a corresponding portion of the P-cadherin molecule have been used to identify the amino acid portions of post-translationally processed cadherin that are required for biological activity. (Nose A. et al. (1990) Cell 61:147). In particular, Nose et al. report that an HAV tripeptide sequence is essential for homotypic cadherin binding. Further, Takeichi report that the amino acid residues flanking the HAV tripeptide sequence also contribute to homotypic binding specificity. (Takeichi M. (1991) Science 251:1451). A review of the literature indicates that research directed to understanding cadherin-mediated adhesion has focussed on efforts to elucidate the mechanism underlying cadherin-mediated homotypic cell adhesion. Little attention has been directed to understanding what, if any, role is played by cadherin in heterotypic cell-to-cell adhesion.
While it has been known for some time that integrins and other adhesion molecules function in immune system modulation, e.g., by playing a role in the adhesion of peripheral lymphocytes to endothelium and in homing to lymph nodes, relatively little is known regarding the mechanism by which lymphocytes home and transmigrate through the vascular endothelium to specifically target tissue locations. In particular, little is known about the molecules that function in mucosal T lymphocyte homing and adhesion, the subset of the general immune system which includes the lymphocytes which populate the gastrointestinal, genito-urinary and respiratory tracts, and the mammary glands. (see, Cepek, K. et al., (1993) J. Immunol. 150, 3459-3470 and references cited therein). In part, the difficulties encountered in cloning the αE subunit (described in U.S. patent application Ser. No. 08/199,776) and in obtaining anti-epithelial cell antibodies which block intra-epithelial lymphocyte adhesion have hindered the identification of an epithelial cell receptor for the intra-epithelial lymphocyte αEβ7 integrin. An incomplete understanding of the role played by cytokines in modulating αEβ7 expression in intra-epithelial cells, also has impeded the identification of a receptor for the αEβ7 integrin.